This invention relates to a method and apparatus for forming an image on a surface, and articles on which such images have been formed.
Thermal transfer printing is a well known printing technique, whereby a dye, resin wax or similar transferable pigments are transferred from a thin carrier film to a receiver media by means of thermal impulses from a thermal array printhead. The pigments may be any colour but are commonly black, yellow, cyan or magenta.
In this process, the thermal array printhead comprises a linear array of several hundred small heater elements, in intimate contact with the carrier film, the pigmented surface of which is pressed against the receiver media.
When selected heater elements on the surface of the thermal printhead are given short pulses of electrical energy, this raises their surface temperature to a value where pigment is transferred from the carrier film to the receiver media to form a pixel or coloured spot on the receiver media. Following this transfer, the carrier film and receiver film are moved relative to the thermal printhead, by a distance equivalent to the diameter of the spot. By a process of repeating this sequence of heat impulses followed by media movement, any desired image is built up. The cycle times for printing a linear array of pixels or colour spots is generally five milliseconds or less. Half tone images are produced by printing a dither pattern of dots and spaces. True photographic quality images are produced by controlling the energies of the heat impulses in conjunction with a suitable diffusion type of dye film. This type of printing is known as dye sublimation or dye diffusion printing.
In certain applications, such as the printing of colour images onto PVC cards for identity purposes, it is necessary to protect the images from damage due to abrasion and wear. The most common method is for a thin, transparent plastic coating to be applied to the printing media. A special plastic overlay film is used, which consists of a thin carrier film coated with a layer of a clear plastic material. The said material may be transferred to the top surface of the printed receiver media by means of a heated roller, or by means of a thermal printhead, in the same manner as the pigment layer is transferred as described above. Essentially all of the plastic material is transferred to the surface of the receiver media, which results in a glossy finish protective overlay, of maximum thickness, to protect the underlying image.